A wide variety of processes use radial or horizontal flow reactors to effect the contact of a compact bed of particulate matter with a fluid and in particular a gaseous stream. These processes include hydrocarbon conversion, adsorption, and exhaust or flue gas treatment. In most of these processes, contact of the particulate material with the fluid decreases the effectiveness of the particulate material in accomplishing its attendant function. In order to maintain the effectiveness of the process, systems have been developed whereby particulate material is semi-continuously withdrawn from the contacting zone and replaced by fresh particulate material so that the horizontal flow of fluid material will constantly contact a compact bed of particulate material having a required degree of effectiveness. A moving bed system has the advantage of maintaining production while the catalyst is removed or replaced. Typical examples and arrangements for such systems can be found in U.S. Pat. Nos. 3,647,680; 3,692,496; and 3,706,536; the contents of each of which are hereby incorporated by reference. A good example of the way in which moving bed apparatus has been used for the contacting of fluids and solids is found in the field of petroleum and petrochemical processes especially in the field of hydrocarbon conversion reactions. Many hydrocarbon conversion processes can also be effected with a system for continuously moving catalyst particles as a compact column under gravity flow through one or more reactors having a horizontal flow of reactants. One such process is the dehydrogenation of paraffins as shown in U.S. Pat. No. 3,978,150, and another such process is the dehydrocyclodimerization of aliphatic hydrocarbons.
Another well-known hydrocarbon conversion process that uses a radial flow bed for the contact of solid catalyst particles with a vapor phase reactant stream is found in the reforming of naphtha boiling hydrocarbons. This process uses one or more reactors. Typically, the catalyst particles enter the top of a first reactor, flow downwardly as a compact column under gravity flow, and are transported out of the first reactor. In many cases, a second reactor is located either underneath or next to the first reactor. Catalyst particles again move through the second reactor as a compact column under gravity flow. After passing through the second reactor, the catalyst particles may pass through additional reactors before collection and transportation to a regeneration vessel for the restoration of the catalyst particles by the removal of coke and other hydrocarbon by-products that accumulate on the catalyst in the reaction zone.
In the reforming of hydrocarbons using the moving bed system, the reactants typically flow serially through the reactors. The reforming reaction is typically endothermic so the reactant stream is heated before each reactor to supply the necessary heat for the reaction. The reactants flow through each reactor in a substantially horizontal direction through the bed of catalyst. The catalyst particles in each reactor are typically retained between an inlet screen and an outlet screen that together form a vertical bed and allow the passage of vapor through the bed. In most cases the catalyst bed is arranged in an annular form so that the reactants flow radially through the catalyst bed.
Experience has shown that the horizontal flow of reactants through the bed of catalyst can interfere with the gravity flow removal of catalyst particles. This phenomenon is usually referred to as hang-up or pinning and it imposes a constraint on the design and operation of reactors with a horizontal flow of reactants. Catalyst pinning occurs when the frictional forces between catalyst particles and the outlet screen that resist the downward movement of the catalyst particles are greater than the gravitational forces acting to pull the catalyst particles downward. The frictional forces occur when the horizontal flow of vapor passes through the catalyst bed and the outlet screen. When pinning occurs, it traps catalyst particles against the outlet screen of the reactor bed and prevents the downward movement of the pinned catalyst particles. In a simple straight reactor bed, or an annular bed with an inward radial flow of vapors, pinning progresses from the face of the outlet screen and, as the vapor flow through the reactor bed increases, it proceeds out to the outer surface of the bed at which point the bed is described as being 100% pinned. Pinning between the outlet screen and the outer surface occurs when the frictional forces between catalyst particles that resist the downward movement of the catalyst particles are greater than the gravitational forces acting to pull the catalyst particles downward, thereby trapping catalyst particles against pinned catalyst particles. Once pinning has progressed to the outermost portion of the catalyst bed, a second phenomenon called void blowing begins. Void blowing describes the movement of the catalyst bed away from an inlet screen by the forces from the horizontal flow of vapor and the creation of a void between the inlet screen and an outer catalyst boundary. The existence of this void can allow catalyst particles to blow around or churn and create catalyst fines. Void blowing can also occur in an annular catalyst bed when vapor flows radially outward through the bed. With radially outward flow, void blowing occurs when the horizontal flow of vapor creates a void between the inner screen and the inner catalyst boundary. Therefore, high vapor flow can cause void blowing in any type of radial or horizontal flow bed.
The trapping of catalyst particles within a reactor bed that is designed to move continuously causes some catalyst particles to remain in the bed for a longer time than other catalyst particles that still move freely through the bed. As the trapped catalyst particles deactivate and thereby become less effective at promoting the desired catalytic reactions, the reactor bed as a whole exhibits a performance decline, which imposes a direct loss in the production of the desired product. In addition, the production of fines can pose a number of problems in a continuous moving bed design. The presence of catalyst fines increases the pressure drop across the catalyst bed thereby further contributing to the pinning and void blowing problems, can lead to plugging in fine screen surfaces, contributes to greater erosion of the process equipment, and in the case of expensive catalysts imposes a direct catalyst cost on the operation of the system. Further discussion of catalyst fines and the problems imposed thereby can be found in U.S. Pat. No. 3,825,116, which also describes an apparatus and method for fines removal.
Where possible, horizontal or radial flow reactors are designed and operated to avoid process conditions that will lead to pinning and void blowing. This is true in the case of moving bed and non-moving bed designs. Apparatus and methods of operation for avoiding or overcoming pinning and void blowing problems are shown in U.S. Pat. Nos. 4,135,886; 4,141,690; 4,250,018; and 4,567,023, the contents of each of which are incorporated herein by reference. To avoid process conditions that lead to pinning, it has been the practice for many years to operate reactors of continuous and semi-continuous moving bed designs by maintaining the flow of vapor through the bed of catalyst at a rate that is below the rate that will pin catalyst when the bed is stagnant. This rate is referred to herein as the stagnant bed pinning flow rate.
As explained in further detail in the detailed description below, the stagnant bed pinning flow rate is the fluid rate that prevents at least a portion of the particles in a bed, which is initially stagnant, from moving downward when particles are withdrawn from the bottom of the bed. In the past, the stagnant bed pinning flow rate has been estimated using a theoretical analysis of the mechanics within the stagnant bed of particles. A suitable analysis is described in the article written by J. C. Ginestra et al. at pp. 121–124 in Ind. Eng. Chem. Fundam. 1985, 24. The inputs to this analysis are the physical properties of the particles; the condition of the particle bed; the geometry of the particle bed and of the screens and walls retaining the bed; the physical properties of the screens and walls, if any, retaining the bed; the physical properties of the fluid; and the operating conditions of the bed. The condition of the particle bed takes into account the solid fraction of the particle bed, the particle-screen static friction factor, and the particle-particle static friction factor. Confirmation of this estimate of the stagnant bed pinning flow rate can be obtained by experiment. The experimental apparatus is a vertically-extended bed of particles between an inlet screen and an outlet screen, with an inlet at the top of the bed and an outlet at the bottom of the bed for downward flow of particles. The apparatus also has a fluid inlet and a fluid outlet for cross-flow of a fluid. While the particle outlet is closed, the particle bed is formed by introducing particles through the particle inlet in the same manner as particles are introduced through the inlet when particles are flowing downward through the bed. Only a short time after loading in order to ensure that the solid fraction of the particle bed is essentially the same as when the particles are loaded, the fluid flow rate through the bed of particles is started at a relatively high rate such that, once downward flow of particles begins, a substantial portion (i.e., about 25–50%) of the particles within the bed is pinned. Then, the particle inlet and outlet are opened, so that particles flow downward through the bed. Next, the flow rate is reduced stepwise, with each reduction in flow unpinning some of the particles that had been pinned, until the final downward step in flow rate results in no pinning of any of the particles. The stagnant bed pinning flow rate can be determined by averaging the penultimate and final flow rates. The precision of the measurement of the stagnant bed pinning flow rate can be improved by decreasing the step size between the penultimate and final flow rates.
Many moving bed design reactors in commercial plants around the world have operated for years and even decades at vapor rates below the stagnant bed pinning flow rate described in the preceding paragraph and thereby have successfully avoided any pinning problems. Despite thousands of successful plant-years of operation, catalyst pinning, although rare, can occasionally occur in a radial flow reactor of the continuous or semi-continuous moving bed design. When pinning does occur, a short procedure is typically used to “unpin” any pinned catalyst. First, the vapor flow rate is decreased significantly below, i.e., typically at least 10–50% below, the stagnant bed pinning flow rate, and then the flow rate is increased to a rate which is less than the stagnant bed pinning flow rate. The catalyst withdrawal rate may be stopped or decreased, either before, simultaneously with, or after the reduction in vapor rate. If the catalyst withdrawal is stopped or decreased, then it is usually restarted or increased prior to increasing the vapor flow rate. In cases of severe pinning where this short procedure is unsuccessful, the vapor flow is stopped and the pinned catalyst is manually removed from the bed.
Occasionally, commercial reactor beds that are designed to move continuously stop moving and come temporarily to rest. This happens intentionally when the reactor bed is designed for semi-continuous catalyst withdrawal. Depending on the design and operation of the commercial plant, these periods of time at rest can be in the range of from as low as 1–2 minutes to as high as 6–12 months, but they are commonly in the range between 10 minutes and 1 hour. When catalyst flow is resumed, catalyst in these reactor beds does not become pinned, as evidenced by the absence of any symptoms of pinning.
Methods of operation for increasing the vapor flow rate in moving bed processes while avoiding pinning problems are sought.